Victor Dricks, a spokesman for the NRC, said a small release could have occurred because the leaking component at San Onofre’s Unit 3 reactor vents into a building outside the reactor’s towering containment dome. That building is not sealed or pressurized, he said.

“We know that it vented some radioactive gases into the auxiliary building because that’s what triggered the radiation alarm that told them they had a leak,” Dricks said.

Whether the gas got out of the unsealed building is not known. Dricks said that any gas that might have escaped would be considered a “very low-level release.”

Edison said in a statement released Wednesday evening that “sensitive monitoring instruments at Unit 3 continues to show no change in radiation levels that would be detectable off-site.”

Edison took its Unit 3 reactor off-line at 5:31 p.m. Tuesday after detecting a leak in one of its two steam generators.

In a news release, the company said it believes at least one of the 9,727 thin tubes inside the generator burst, allowing radioactive reactor coolant to mix with nonradioactive water used to make the steam that generates power at the plant.

However, on Wednesday Edison appeared to retreat from its initial statement Tuesday evening declaring that no amount of radioaction had been released into the atmosphere.

Edison spokesman Gil Alexander told the Associated Press on Wednesday that he agreed with Dricks’ statement that a tiny amount of radiation could have escaped.

“I can’t speak for the NRC, but we would agree that there might have been an insignificant or extremely small release,” Alexander said.

Neither the NRC nor Edison were able to supply the amount of radiation detected in the auxiliary building. The NRC is continuing to investigate Edison’s response to the incident.

Alexander also said Wednesday that radioactive water was passing through a crack inside the generator at a rate of 80 to 100 gallons per day when Edison began shutdown procedures.

He said engineers spent Wednesday drawing up plans on how they would pinpoint the leak while they waited for the reactor to cool enough to enter the Unit 3 containment dome.

Until engineers can enter the Unit 3 structure and begin probing each individual tube, they won’t know how extensive the damage is, officials said.

Edison did not respond to a question Wednesday about who will be financially responsible for fixing the break, since the units are nearly brand new.

The plant’s other reactor, Unit 2, was already shut down for refueling and maintenance.

When both of its reactors are running at full power, San Onofre generates 2,200 megawatts of electricity, enough to power 1.4 million average-size homes.

The shutdown of the Unit 3 reactor means the plant is generating no electricity, but power grid operators said they were able to cope with the loss.

The region had plenty of reserve generation to keep the lights on for the next few weeks, barring a sudden change in weather, said Stephanie McCorkle, spokeswoman for the California Independent System Operator, the agency responsible for balancing electric supply and demand in California.

The state’s power grid is designed to withstand high energy use typically seen in late summer, when air conditioners and pool pumps run at maximum capacity.

Since it’s February and demand is lower, McCorkle said the region easily had enough power for a peak demand at 8 a.m. and expected to have no problems at 7 p.m. peak.

Other power plants are also on standby if needed, McCorkle said. By law, the operator keeps a 7 percent reserve of electricity.

“Electricity is not something you can bottle up or store somewhere,” she said. “It is consumed the instant it is produced. Because you’re basically matching supply and demand in real time, we have to maintain a certain amount of operating reserves. These are plants in stand-by mode that can ramp up quickly if something happens.”

McCorkle said she couldn’t name the standby plants because it’s considered “market sensitive information.” However, the Encina Power Station in Carlsbad, and the Palomar Energy Center in Escondido both have capacity to help keep electricity flowing locally.

San Onofre just finished replacing both steam generators attached to Unit 3. Many other plants of similar design have conducted similar upgrades, but none, according to the NRC, has had a rupture after being installed.

The leak is not Edison’s first problem with the new steam generators.

In September 2009, Edison met with NRC officials to discuss weld defects that were detected in both of the 640-ton crucibles.

The utility notified the NRC in August 2009 that the replacement generators for Unit 3 had developed cracks in a weld that connects a 5-inch-thick steel plate that supports each generator’s innards.

Inspections found that use of a special metal-gouging tool caused the welds to become brittle and crack. Mitsubishi Heavy Industry, the company that built the generators for Edison, fixed the cracked welds, and they passed NRC inspection.

It was not known Wednesday whether the leak occurred in that area.

Some groups have recently criticized the way Edison has handled releasing information regarding problems.

In November, nuclear watchdog and environmental groups criticized plant operators for taking more than an hour to notify the public of an ammonia leak in a storage tank that prompted the evacuation of some workers. There was no danger to the public, the company said at the time.

Daniel Hirsch, who lectures on nuclear policy at UC Santa Cruz, said he was concerned that this week’s water leak occurred with recently installed equipment.

“Edison has historically not been candid about the problems at San Onofre. That lack of transparency causes tremendous distrust and increases risk,” Hirsch said.

“It makes one wonder about the quality assurance for the replacement equipment,” he added. “This is not due to old equipment breaking, but new equipment that wasn’t up to snuff in the first place.”

Dave Lochbaum, a former nuclear engineer turned industry critic who works with the nonprofit Union of Concerned Scientists, said Wednesday that a nuclear plant’s steam generators have long been a place where radiation releases are a possibility.

This is because a reactor’s radioactive primary coolant water, which continuously circulates through the reactor’s core, passes through thousands of relatively thin metal tubes that are surrounded by clean water. That water becomes steam and leaves a plant’s protective containment structures in order to spin turbines that generate electricity.

“Those tube walls are about the thickness of a dime,” Lochbaum said.

Utilities like Edison are expected to scan the tubes inside a reactor’s steam generators every two years when a reactor is refueled.